The present invention relates to a pupil divisional type focusing position detection apparatus.
Conventionally, there is known a focusing position detection apparatus utilizing a photoelectric element whose resistance becomes maximum when the image of an object is focused on the photoelectric element. In another conventional focusing position detection apparatus, double images of the object are respectively projected onto different photoelectric elements and the difference between the electric currents which flow through the respective photoelectric elements is measured and when the image of the object is focused, the difference of the photoelectric currents becomes minimum.
In these focusing position detection apparatuses, the focusing position itself can be determined. However, when the object is out of focus, it cannot be determined whether the focused position is in front of the film surface (hereafter referred to as a front focusing state) or the focused position is behind the film surface (referred to as a rear focusing state). Therefore, in these apparatuses, it is required that a prescanning be initiated from a predetermined direction, for example, from close range to infinite range so that the focusing position is searched and detected and then the photographing lens is moved to the detected focusing position. This prescanning step, however, is laborious.
A focusing position detection apparatus which does not require the prescanning can be proposed. The principle of the focusing position detection apparatus will now be explained by referring to FIG. 1 through FIG. 6.
In the focusing position detection apparatus, as shown in FIG. 1, rays of light are caused to enter a pair of self-scanning type photoelectric element arrays 3, such as CCD (Charge Coupled Device) type image sensors and MOS type image sensors, from a peripheral portion of an exit pupil of a photographing lens 1 through a group of small lenses 2 such as lenticular lenses and fly-eye lenses so that the focusing position is detected from the difference of the phases of the output signals of the photoelectric element arrays 3. This type of the focusing position detection apparatus is referred to as a pupil divisional type focusing position detection apparatus since rays of light coming from two divided portions of a photographing lens are utilized in detecting the focusing position. The focusing position is detected as follows: When the image of an object is in focus as shown in FIG. 1, the divisional rays of light correctly enter a pair of photoelectric elements 3a-1 and 3a-2 from the photographic lens 1. Therefore, the phases of the outputs of the respective photoelectric elements 3a-1, 3a-2 are in agreement with each other. When the image is focused in front of the focusing plane, that is, in the so-called front focusing state, the divisional rays of light from the photographing lens 1 respectively enter another pair of photoelectric elements 3b-1, 3c-2, so that the phases of the output signals of the pair of the photoelectric elements 3b-1, 3c-2 are shifted from each other as shown in FIG. 4. On the other hand, in the case of a rear focusing state as shown in FIG. 5, a divisional ray of light l.sub.1 enters a photoelectric element 3c-2, which is indicated by a white block, although, in the focusing state, the divisional ray of light l.sub.1 should enter the photoelectric element 3a-1, which is indicated by a solid black block in FIG. 5. Furthermore, in the back focusing state, another divisional ray of light l.sub.2 enters the photoelectric element 3b-1, which is indicated by a black block. As a result, the phases of the output signals of the photoelectric elements 3c-1, 3b-2 are reversed as shown in FIG. 6 in contrast with the case in FIG. 4. Thus, by detecting the difference of the phases of the output signals of the photoelectric elements, it can be determined whether the image of an object is in focus or in the front or rear focusing state.
In the case of this sort of the focusing position detection apparatus, the detection accuracy of focusing position is usually set at its maximum accuracy and therefore the focusing position is deviated by a slight movement of either the taking lens or an object. In particular, when the accuracy of detecting focusing position is high, great care has to be taken when the taking lens is moved manually to determine the in-focus position correctly in accordance with the detected result, while watching the focus indication within a viewfinder of a camera. In a camera of the type in which the taking len is automatically moved by a motor in accordance with the detected result, it is required that the motor be always rotated. This needs laborious steps.
Furthermore, since the shooting aperture is usually reduced when taking pictures, F-number of the taking lens is usually set so as to correspond to the reduced shooting aperture. Therefore, if the detection accuracy is fixed, the focusing is performed at a higher accuracy than it needs.
Furthermore, in the pupil divisional type focusing position detection apparatus, pairs of photosensors are usually disposed as shown in FIGS. 7 and 8 and a pupil divisional angle .theta..sub.F is fixed. Since the pupil divisional angle .theta..sub.F corresponds to the F-number of the taking lens in the relationship of F.sub.NO =(1/2 sin .theta..sub.F) when the taking lens is set at an indefinite focusing.infin. if the pupil divisional angle .theta..sub.F is constant, the F-number is also constant. It follows that no taking lens having a different F-number can be used in the pupil divisional type focusing position detection apparatus as shown in FIGS. 7 and 8. However, interchangeable lenses having different F-numbers are frequently used in practice in cameras and F-number is relatively changed in accordance with the change of the shooting aperture by adjusting the diaphragm. Therefore, when a mounted taking lens is replaced with a taking lens having the same focal length as that of the previous taking lens and having a larger aperture relative to a predetermined F-number, ordinarily the pupil divisional angle .theta..sub.F is increased and accordingly the detection accuracy is also increased. However, since the pupil divisional angle .theta..sub.F is constant, the detection accuracy is not increased in this case. Furthermore, when a taking lens having a smaller aperture relative to the predetermined F-number is mounted or when the shooting aperture of the taking lens is reduced, divided rays of light for measuring the distance of objects are not received correctly by the corresponding photosensors, so that it becomes impossible to detect the focusing position accurately. When closeups are taken by moving the taking lens forwards, F-number is relatively increased so that detection of the focusing position becomes impossible again.